Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2015

Schedule of Stock-Based Compensation Expense

The following stock-based compensation amounts were recognized for the periods indicated (in thousands):

	Three Months Ended March 31,
	2015			2014
Research and development		$	810		$	317
General and administrative			821			295
Total stock-based compensation expense		$	1,631		$	612

Schedule of Fair Value Option Award Estimated on the Date of Grant Using the Black-Scholes Option-Pricing Model

The fair value of each option award is estimated on the date of grant using the Black-Scholes option-pricing model using the assumptions in the following table:

	Three Months Ended March 31,
	2015	2014
Expected dividend yield	0%	0%
Expected volatility	74%	67 %
Risk-free interest rate	1.6% - 2.0%	2.1% - 2.3%
Expected term	6.25 years	6.25 years

Schedule of Stock Option Activity

The following table summarizes stock option activity under the Plan during the three months ended March 31, 2015:

	Shares			Weighted- Average Exercise Price		Weighted-Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value (in thousands)
Outstanding, December 31, 2014		3,572,116		$	11.40		7.3
Granted		39,975			32.91
Exercised		(105,820	)		2.57
Forfeited or expired		(16,618	)		17.59
Outstanding, March 31, 2015		3,489,653			11.88		7.2		$	68,188
March 31, 2015:
Exercisable		1,697,219			3.80		5.3			46,834
Vested and expected to vest		3,269,812			11.50		7.1			65,120